1. Field of the Invention
The present invention relates to a scanning probe microscope, and more particularly, to a cantilever for a scanning probe microscope (SPM), which minimizes an inner coupling.
2. Background of the Related Art
In general, of the SPM, AFM (atomic force microscope) and the like uses a micro-machined cantilever. The cantilever is flexible in up and down directions, and has a probe with a sharp tip of a size of a few atoms.
When the probe is brought close to a surface of a test piece, there is an attractive or repulsive force exerting between the atoms at the tip of the probe and the atoms on the surface of the test piece according to a distance between the two. The AFM is a device that senses the force, and, currently, the principle is also applied to nano-lithographies, data storage systems, and the like.
Of the various operation modes of the AFM, the repulsive force is used in a case of a contact mode, and, though the force is very fine in a range of approx. 1-10nN, the force bends the cantilever as the cantilever is also very sensitive.
In order to measure an up or down direction bending of the cantilever, a laser beam is directed to the cantilever, and an angle of the laser beam reflected at a top surface is measured by a photodiode.
Such a movement of the tip of the cantilever is fed back to a piezotube driver, to sustain the bending of the cantilever, and, also fix the distance between the tip and the test piece, thereby permitting measurement of a shape of the test piece.
However, the sensing device employing the laser and the photodiode has disadvantages of requiring complicated and precise devices. Consequently, for solving the problem, even a method is suggested, in which a sensing device is employed, which has an integrated piezoresistor on the cantilever.
Moreover, the AFM requires too much time in measuring the test piece, which is the greatest obstacle in etching devices and data storages having the AFMs applied thereto. A major reason of the AFM requiring much time in measuring the test piece is a poor mechanical responsive efficiency, and a very low resonance frequency. Therefore, for solving the problem, even a technology is suggested, in which, instead of the piezotubes, piezoelectric actuators are integrated on a base of the cantilever.
Recently, a cantilever for an AFM is suggested, in which piezoresistor sensors and piezoelectric actuators are integrated. FIG. 1A illustrates a perspective view of a related art cantilever having piezoresistor sensors and piezoelectric actuators integrated thereon, FIG. 1B illustrates a section across a line I—I in FIG. 1A, FIG. 1C illustrates a section across a line II—II in FIG. 1A, and FIG. 1D illustrates a plan view of FIG. 1A.
Referring to FIGS. 1A-1D, the related art cantilever 7 of silicon is provided with a sensing part 6 having a coat of boron for serving as a piezoresistor, and a sensing signal transmitting part 5 for transmitting an electric signal to the piezoresistor.
The silicon cantilever is heavily doped with boron so that a mechanical stress occurred at the actuator does not cause an unnecessary electric signal at the sensing signal transmitting part 5.
An insulating film 4 is formed on the silicon cantilever having the sensors integrated thereon for electric insulation, and a ferroelectrics capacitor, an actuator operation part, is formed thereon in a structure of a lower electrode 3—a ferroelectrics 2, an upper electrode 1.
Referring to FIG. 1B-1C, a Cpzt represents an equivalent capacitor of the ferroelectrics in the actuator, Cox represents a parasitic capacitor composed of the lower electrode 3—the insulating film 4—the silicon cantilever 7, Cj represents a parasitic capacitor caused by a depletion layer formed between the p-type sensing signal transmitting part 5 and the n-type silicon cantilever 7, Rpt represents a resistance of the lower electrode 3 of the actuator, Rpiezo represents a resistance of the sensing part 6, and Rp+ represents a resistance of the sensing signal transmitting part 5. A π-model, which is used frequently in an equivalent model, is applied in preparing the foregoing model, and Cox and Cj are divided into two, to have Cov/2 and Cj/2.
The foregoing related art cantilever can not avoid electric coupling caused by the parasitic capacitors Cov1 and Cov2 formed at the lower electrode 3 of the ferroelectrics capacitor and the sensing signal transmitting part 5 and the resistance Rp+ at the sensing signal transmitting part.
In general, in the coupling, there are a mechanical coupling, and an electric coupling. In the mechanical coupling, a stress occurred by a mechanical action of the actuator is transmitted to the silicon cantilever, and the stress is converted into an electric signal by the piezoresistive phenomenon of the silicon, and acts as a noise of the electric signal. In the electric coupling, the parasitic capacitors, and parasitic resistors between electric signal lines cause crosstalk, to form an electric noise. Since such noises are very high compared to a voltage (a few μV−a few hundreds of μV) sensed at a sensor, there have been many researches for minimizing the noise.
Referring to FIGS. 1A-1D in the related art, though elimination of the mechanical coupling is simple by heavy doping of the signal line of the piezoresistor running under the piezoactuator with impurity ions, elimination of the electric coupling is difficult.
Therefore, for elimination of the electric coupling, either a correcting circuit is provided separately, or a complicated circuit and a lock-in amplifier are fitted to a measuring terminal.
However, because those additional devices act as fatal defects in view of degrees of integrity and speed when the cantilever is used in other field of application, and, particularly, pushes up unit cost of the AFM, a cantilever is in need, which can prevent the electric coupling between the sensor signal and the actuator signal by using a more simple method.